General Construction Equipment

Strength—Again?

Reader Jeffrey Cannon, principal, Kleinfelder, Sacramento, Calif., asked about the six diagrams showing break configurations of concrete tested cylinders, given in Figure 2, “Sketches of Types of Breaks” in ASTM C39, “Compressive Strength of Cylindrical Concrete Specimens.” ASTM C39 requires the fracture type to be reported if it's not the usual cone (or hourglass configuration) but does not explain why that data is needed or however meaningful the information.

Not highly publicized is the “Manual of Aggregate and Concrete Testing” at the end of Volume 04.02. The Manual describes factors that affect results obtained using ASTM testing procedures. In Section 10 are discussions about C39 that comment on the type of fractures that help decipher contributors to bad testing and attendant low strengths.

The ideal fracture pattern is described as an hourglass. If this configuration is not attained, then deviations mean something is awry, you are not getting all that can be obtained, and strength is actually higher than determined. You should look back at improprieties because of bad specimen fabrication, poor specimen configuration, nonflat ends, capping and cap improprieties, nonconformance of testing machines, and appurtenances. All of these and more detailed advice are provided in the Manual.

The Manual gives two examples of poor fracture patterns: the first presumably resulting from an overly thick cap; and the second “due to incorrect testing procedures,” which are probably from eccentric loading that put one cylinder side in compression and the other side in tension, creating a bending moment rather than pure compression. Jim Shilstone, a fervent advocate of good aggregate grading, reported that significant amounts of vertically oriented elongated coarse aggregate particles cause vertical fractures and low strengths. Failure patterns provide insight into specimen and test conditions that cause atypical crack patterns.

The hourglass configuration comes from a concept that Bernie first heard about in a structural geology class. It deals with a sphere that is put under vertical compression wherein it distorts laterally (gets shorter and fatter) and looks like a symmetrical egg. The stress creates strain (which allows measurement of the distortion) and, eventually the stress is released when shear forces cause rupture and consequent cracks. The crack pattern that evolves when stress from the now-stressed and strained ellipsoid is released has an hourglass configuration, just like a properly made, properly configured, and properly tested concrete cylinder should.

Distorted configurations deserve attention even though derived strengths meet specification requirements. When strengths are low, or getting progressively lower, questions that arise usually fall into three categories: testing machine and ancillary equipment nuances; specimen manufacture, curing, preparation, and configuration; and concrete-making material deficiencies. Here are two examples of needless agony created by low strengths.

Example 1. Variable and low flexural beam strengths stopped construction of an airport runway, almost resulting in removal and replacement of the concrete. Flexural strength tests are sensitive to a variety of factors, such as improper curing and handling. A petrographic study of the low-strength beams revealed concrete proportions as designed, and that essentially all coarse aggregate particles in “planes” of test-induced fractures were transected. These observations led to examination of testing equipment, which was found faulty.

Example 2. Compressive strengths of CMU couplets removed from exterior wall construction were lower than specified. Failures were at corners that sheared off, and vertical, indicating that there was erratic distribution of the compressive load due to variable and overly thick caps necessitated because of uneven block bedding surfaces—resulting in the lower-than-actual strength. Evaluations of the improper fracture pattern led to properly prepared specimens and truer, higher strength results.

So remember the six break diagrams in ASTM C39 and review the “Manual of Aggregate and Concrete Testing,” which discusses the influencing factors of ASTM concrete testing. Also pay attention to strength-test break configurations and, if compressive breaks, remember strain-ellipsoid concepts. It may help relieve your stress.